This invention relates to applications of imaging techniques to inspect for submicron size particles or other artifacts on surfaces, typically planar surfaces under preparation for integrated circuit construction, to determine surface cleanliness. Particulate contamination and defects are a critical problem in the manufacture of integrated circuits.
It is an industry goal to achieve inspection precision with a lower detection limit (LDL) for particles and other defects in a size of about 0.06 microns. Current instruments are capable of particle inspection in the range of 0.1 to 0.08 microns with 95% count efficiency, as determined by standard calibration techniques using polystyrene spheres. Inspection results bettering these accuracy and resolution levels are extremely difficult to achieve because of the barriers of the physical resolution limits of the illumination source and because of the sheer time involved in inspection of all locations on a proportionately large surface.
Using an instrument available for example from KLA Instruments to inspect at 0.1 micron LDL resolution requires on the order of four minutes to inspect a single 200 mm diameter wafer. Inspection at half that LDL may require as much as four times as long, and the optical systems of conventional inspection art simply cannot resolve particles at or below the particle size limit. Long inspection times require that the inspection be made only infrequently and typically independent of the manufacturing process or at least after the completion of many of the processing steps. In fact, inspection times of more than about 45 seconds are considered impractical in the typical cluster tool semiconductor processing system unless some form of queuing is utilized, often making the use impractical. Since a single wafer which is defective may be valued in the tens or hundreds of thousands of dollars, there is a high premium for accurate and rapid inspection.
In order to understand the special problems related to submicron defect measurement, the following information is offered, along with a discussion of known inspection techniques. The following discussion will be focussed on particle detection but the measurement technology can be applied to other surface imperfections such as scratches, digs, etc, and others.